Flat panel displays (FPDs) ranging from small displays to large displays are widely used today. Still now, it is pointed out that LCDs need to be improved in the contrast when seen in an oblique direction, the luminance uniformity, and the luminance itself.
Polymer films are conventionally used for STN (super twisted nematic) or other types of liquid crystal displays for the purpose of compensating for a phase difference so as to realize color compensation, viewing angle enlargement and the like. LCDs using thin film transistors (TFTs) have improved image quality as compared with the STN type LCDs, but still have the viewing angle problem that the image quality when seen in an oblique direction is different from the image quality when seen from the front. Therefore, for example, the retardation films are optimized to be used for such LCDs. Such retardation films are also used for reflection type LCDs in order to improve the contrast.
In accordance with the recent increase in the screen size, it is now indispensable to enlarge the viewing angle of LCDs. In addition to a demand to enlarge the viewing angle through technological development on LCD panels, there is a very strong demand to enlarge the viewing angle by use of a retardation film. Thus, a retardation film of a higher level of performance is desired.
One cause which declines the viewing angle characteristic of an LCD is an influence of the birefringence of the liquid crystal itself, which is exhibited when light passes through a LCD cell. When linear polarization provided by a polarization plate passes through the LCD cell, the polarization performance is not maintained and a part of the light leaks. Due to this, phenomena such as contrast decline, color tone change and the like, which are not favorable to display occur. What is required of a retardation film is to compensate for this variance in birefringence, which is caused depending on the viewing angle of the LCD.
Recently, vertical alignment nematic liquid crystal displays (VA-LCDs), IPS-LCDs, OCB-LCDs and the like have been developed in order to improve the viewing angle characteristic. These displays are realized as a result of development on the mode of the liquid crystal itself, which is made in order to enhance the contrast by enlarge the viewing angle of the display. However, in order to fulfill a recent need for a higher image quality with a wider viewing angle in a larger liquid crystal display, the function of a retardation film is indispensable. It is an important issue to obtain a large size of retardation film for a LCD, which has a uniform retardation value, a uniform delay axis direction, and good surface characteristics (durability against damage).
For realizing this, many retardation films have been proposed. For example, Japanese Laid-Open Patent Publication No. 2006-91246 proposes a retardation film which, when used in a liquid crystal display, does not change much the color tone of a displayed image and contributes to the enlargement of the viewing angle. Japanese Laid-Open Patent Publication No. 2004-117625 proposes a retardation film which, when stretched by a tensile force application, provides the following differential spectrum of infrared absorption before and after the stretching: the absorbance change ratio A of a carbonyl group stretched for 15 seconds to 1 hour, and the absorbance change ratio B of the carbonyl group stretched for 10 minutes to 1 hour, have the relationships of 1.2≦A≦2.0 and B≧1.1.
Japanese Laid-Open Patent Publication No. 2005-77963 proposes that polycarbonate having a high refractive index, a low Abbe value, a high glass transition temperature (Tg), and a photo-elastic constant equal to or lower than a specific value is promising to produce a retardation film which has a high size stability against time and temperature change and does not change much the optical properties thereof against external stress.
However, these proposals do not clearly define the optical characteristics of the film and so are limited in applications. Especially, these proposals do not fulfill the recent demand for a display device providing a high image quality. Especially in order to fulfill the recent demand for a high level display, a retardation film having controlled refractive indices in three dimensions is important. Such refractive indices are appropriately defined by Nz factor. Nz factor=(nx−nz)/(nx−ny) (in the expression, nx and ny are each the main in-plane refractive index, and nz is the main thickness-direction refractive index.)
Several such retardation films having a controlled Nz factor have been proposed. For example, Japanese Laid-Open Patent Publications Nos. 2005-62673 and 2005-62671 each propose a retardation film having an Nz factor of 0.5 to 2.0. Japanese Laid-Open Patent Publication No. 2004-309617 proposes a retardation film having an Nz factor fulfilling 1.00≦Nz factor<1.35. Japanese Laid-Open Patent Publication No. 2006-58540 proposes a retardation film having an Nz factor fulfilling 0.1≦Nz factor≦0.9.
All these retardation films have an Nz factor no more than 2. No retardation film having a larger Nz factor with more controlled molecular alignment has been proposed.
In the meantime, in order to improve the image quality of LCDs, it is necessary to improve the luminance and decrease the luminance nonuniformity. As a polymer film, a diffusion film or a diffusion sheet is used. The most important properties required of a diffusion film are to improve the luminance and to diffuse light or to conceal the nonuniformity pattern of the light source or behind the light source.
Conventionally for liquid crystal displays or the like, a surface light source called “edge light type backlight” is used. For a liquid crystal display using such a light source, an optical diffusion film for diffusing light emitted from the light source so as to make the image of the light source invisible is required.
Recently, optical diffusion films are desired to have an appropriate range of haze values and thus have a high optical diffuseness of diffusing light emitted from the light source uniformly on a liquid crystal panel, and also to have a high luminance, as well as a good light transmittance.
Examples of such an optical diffusion sheet include a sheet having an optical diffusion layer containing polymer beads or inorganic microparticles provided on at least one surface of the film (see Japanese Patent No. 2665301) and a sheet having a convex and concave pattern provided by embossing made on one surface or both surfaces of a transparent plastic film and also an optical diffusion layer containing microparticles on one surface or both surfaces thereof (see Japanese Laid-Open Patent Publication No. 11-337711).